The long term goal of this project is to understand the initial molecular specification of neural crest segmental identify, and its relationship to development of the caudal pharyngeal arches. The proper formation of these arches is critical for normal development of the cardiovascular system. Specifically, we will disrupt normal symmetrical development of pharyngeal arches 3 and 4 by ablating expression of the DNAs binding region of groups of homeotic proteins in the cardiac neural crest. Heart development will be monitored during the process of pharyngeal patterning and after the heart is phenotypically mature. A novel method will be used for ablation of homeotic protein function in the cardiac neural crest. The neural folds encompassing the cardiac region are removed and incubated in antisense oligonucleotides that have been designed to disrupt translation of the homeobox region of the homeotic proteins, such that only truncated homeotic proteins will be expressed. The neural folds are then replaced in a normal host embryo from which the same region of the neural fold has been removed. The advantage of this method are that (1) groups of proteins can be targeted in a single experiment (in transgenic animals it has only been possible to target a single gene), and (2) only a single group of cells, the cardiac neural crest will be affected, such that the other cells involved in cardiovascular development are normal. Patterning of pharyngeal arches 3 and 4 will be followed at various stages of development after the backtransplantation using in situ hybridization and morphometry.